Document DE 41 25 655 A1 discloses a resonator apparatus for electron spin resonance spectroscopy. This prior art resonator apparatus utilizes a dielectric resonator ring. The microwave energy required for exciting the resonance is fed via a coaxial cable terminating in a coupling loop. The coupling loop serves as an antenna located in the exterior area of the resonance ring, and is adapted to be displaced in a direction parallel to the axis thereof. A conventional sample tube, closed at its bottom, is used as the sample vessel, and, therefore, the sample tube contains a solid sample substance.
Therefore, this prior art apparatus, as disclosed, is not suitable for measuring liquid samples.
Document DE 30 29 754 A1 discloses a probe head for electron spin resonance measurements. This prior art probe head, too, utilizes a dielectric resonance ring as a resonant element. However, in this prior art probe head the microwave excitation is effected via a hollow waveguide and a slit iris for coupling microwave energy into the resonance ring.
This prior art probe head, too, is only suited for measuring solid samples.
Document WO 01/36994 A1 discloses a method and an apparatus for investigating the durability of liquid food stuffs by means of electron spin resonance.
According to the described method, the durability of beer is preferably determined by means of electron spin resonance. For that purpose, the prior art apparatus utilizes a sample vessel provided with a plurality of elongate sample areas having a minimum radial dimension. The sample vessel either consists of a solid block having a corresponding plurality of axial bores, or is made up of a bundle of capillaries. The probe vessel extends through a hollow cavity. The liquid sample substance flows through the cavity at a location where the magnetic field strength is at a maximum because all bores or capillaries are located in that area of maximum magnetic field strength.
These prior art apparatuses, however, have the disadvantage that the extremely thin sample channels tend to clog. For example, one individual thin sample channel may be clogged either by a solid particle within the liquid sample substance, or by gas bubbles contained within the sample substance. This may happen mainly in connection with those sample substances having a gas diluted therein, such as, for example, beer or other carbonated food stuffs. In that case, the result of the measurement is substantially affected. Moreover, with these prior art apparatuses the cleaning of the capillary-like sample channels presents a problem.
Finally, it is generally known in the art of electron spin resonance measurements on liquid samples to use so-called “flat cells”, as are used within hollow cavity resonators for the conventional TE102 mode of excitation. The term “flat cell” designates a sample vessel consisting of an originally cylindrical glass tube being compressed over a certain axial length so that only a flat parallelepiped sample volume remains in that flattened area.
Flat cells, however, also have the disadvantage that rinsing and cleaning presents a problem because the sample substance does not flow sufficiently through the lateral corners in the transition between the circular cylindrical and the flat section of the flat cell.
All prior art apparatuses, further, have the common disadvantage that the sample vessels, to the extent as they are configured for a sample substance flowing therethrough, always have to be connected within the experimental setup via special couplings and fittings which conventionally consist of metal and, hence, tend to corrode. Finally, not all prior art sample assemblies are adapted for measurements on liquid samples that are at an elevated temperature of, for example, 60° C.
It is, therefore, an object underlying the present invention to provide an improved resonator apparatus as well as a corresponding method, such that the aforementioned disadvantages are avoided. In particular, the invention shall enable fast, undisturbed and sensitive measurements on liquid sample substances, and allow a quick change between sample substances without needing to change the sample vessel.